Industrial circuit breakers in the larger current ratings are typically provided with plural sets of movable and stationary main contacts, as well as a set of movable and stationary arcing contacts in each breaker pole. The movable main contacts are individually mounted by separate pivotal arms, while the movable arcing contact is mounted by yet another pivotal arm of somewhat greater length such that the movable arcing contact can engage its stationary counterpart positioned within the entry portion of an arc chute. The main and arcing contact arms are ganged together by a carrier which, in turn, is ganged with the movable contact arm carriers of the other breaker poles by a crossbar. The breaker operating mechanism is typically linked to the center pole contact arm carrier pursuant to achieving concerted movements of the movable contacts of all the breaker poles between open and closed circuit positions with respect to their associated stationary contacts.
The movements of the movable main and arcing contacts are preferably coordinated such that, during a circuit interruption, the main contacts separate before the arcing contacts. This is done to transfer at least a major portion of the current to the arcing contacts such that upon their separation an arc will be drawn between the arcing contacts and any arcing at the main contacts will then abruptly terminate. Since this ideally locates the arc well within the entry portion of the arc chute, arc extinction can be readily effected.
Unfortunately, particularly in the case of the high fault current interruptions, major transfer of the current to the still closed arcing contacts does not occur, and consequently substantial arcing at the main contacts cannot be avoided. In fact, the arc drawn between the main contacts as they separate can become so well established that it just stays there, and little or no arcing occurs between the arcing contacts upon their separation. Since the main contacts are not located in the entry portion of the arc chute and thus do not have the benefit of its arcing extinguishing capabilities, the arc can and indeed does considerably damage the main contacts and adjacent conductive parts which are not designed to withstand prolonged arc rooting.
This problem is well recognized and numerous approaches have been proposed with varying degrees of success to move the arc from the main contacts out to the arcing contacts. For example, sources of gases have been utilized to blow the arc off the main contacts. Separate magnetic blowout coils to develop electromotive forces propelling the arc off the main contacts have also been utilized. Both of these approaches, while generally successful, are typically expensive to execute. Elaborate structuring of the circuit breaker internal circuit has been proposed to route the fault current feeding the arc along prescribed paths calculated to develop electromotive forces propelling the arc off of the main contacts. This approach invariably requires an expensive circuit breaker redesign. Another proposal involving the provision of a magnetic arc runner located between the stationary main and stationary arcing contacts for the purpose of attracting the arc off the main contacts, while inexpensive to implement, has not been found to be particularly successful.
It is accordingly an object of the present invention to provide a circuit breaker which is equipped with means for minimizing arcing at its main contacts.
A further object is to provide a circuit breaker of the above character wherein the arc drawn between the main circuit breaker contacts is effectively and expeditiously transferred to the arcing circuit breaker contacts.
Yet another object of the present invention is to provide a circuit breaker of the above-character, wherein the transfer of the arc from the main to the arcing contacts is propelled by the magnetic fields associated with the currents feeding the arc.
An additional object is to provide a circuit breaker of the above character which includes a line strap for mounting the stationary main and arcing contacts and wherein the line strap is uniquely structured to route the current feeding an arc drawn between the main contacts in a manner to develop electromotive forces propelling the arc off the main contacts and out onto the arcing contacts.
Still another object is to provide a circuit breaker of the above character wherein the uniquely structured line strap of the present invention is inexpensive to produce and can be implemented without redesigning the remainder of the circuit breaker.
Other objects of the invention will in part be obvious and in part appear hereinafter.